Well I made this flash animation in hopes to explain piston valves for the most part. Let me know if this helps anyone figure them out. Those who already know, please point out if you spot something wrong.

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<font size =+2><b>Phase Explainations<br></font><font size =+1>Applies to all Models</b></font>
You may need to watch it several times because there are many things going on at once. This assumes you know the relationship between pressure, surface area, and force (pressure * surface area = force)

<b>Phase 0:</b> Basically just the gun in its pre-use state. No pressure in the gun.
<b>Phase 1:</b> An air source is connected behind the piston. Air enters the pilot chamber and the pressure pushes the piston against the rear of the barrel or chamber port.
<b>Phase 2:</b> Air continues to flow through the input and leaks around the outside of the piston or through an equalization hole into the main chamber. Once desired pressure is reached, the input flow is cut off and the gun is ready to fire.
<b>Phase 3:</b> The exhaust valve is opened and the pressure begins to fall in the pilot chamber. The exhaust must exhaust faster than the equilization hole can leak air back into the pilot. Once the force pressing on the back of the piston falls below the force acting on the front of the piston, it begins to slide back. Suddenly there is more surface area exposed on the front of the piston, and the jump in force slams the piston back leaving an opening for air to flow into the barrel to accelerate the projectile.

The exhaust valve is closed and a new projectile loaded. The gun is then back to it's original condition, ready for the cycle to repeat.

The main thing to notice between the different types are the forces caused by pressure. Of course forces could be changed by changing the changing the piston/sealing port diameter, but in the general senario like the one shown, with a constant barrel diameter between all types and a tee slightly larger than the port, the barrel sealing valve will have the same forces as coaxial in the closed position. Barrel sealing will have the highest opening force, coaxial will be in the middle, and chamber sealing will have both a low closing force and a low opening force.
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<font size=+2><b>Coaxial Piston Valve</b></font>
<embed src="http://gbcannon.com/flash/coax.swf" quality="high" bgcolor="#ffffff" width="550" height="350" name="coax" align="middle" allowScriptAccess="sameDomain" type="application/x-shockwave-flash">

Update: Now includes a control panel with play, pause, skip, frame by frame, and slow motion (took forever to figure out).

Update 2: Added labels for both the controls(mouse over) and the gun parts. You may need to Ctrl+F5 to see the updated flash if you viewed this page before. Also added a barrel sealing tee flash and tuned down the forces on the Coaxial flash to be more accurate compaired to the Barrel Sealing Tee flash.

Clide, this is a great visual for those who are having a hard time understanding the finer points of a pilot valve operation. It is simple and best of all interactive. I agree with Freefall on slowing down step three a bit. Maybe add some labels? Someone might have a hard time recognizing the piston, barrel, chamber, pilot chamber? You could put an arrow pointing to the component with a text label in only phase zero.

Did you hear that thump? That was my jaw hitting the table.
I swear, I had a nearly identical idea last night, it came to me in a vivid dream (ask me about dream-state clairvoyance). If I knew anything about flash, I'd have done it myself.

Very nicely done, better than any plain-text explanation could possibly do. I especially like the net force indicator. The only suggestion I can make is that you slow down phase 3 a bit. It's hard to pick up on everything the first time through.

Patduck: if you're talking about filling the chamber after the valve seals, it's all done through the same valve. The pressure just flows through the gap between the piston and wall, or through a small leak port.

Thanks all. I took your advice freefall (kinda) and put in a control panel for better control of the animation. Oh, and didn't see your edit until now plasticex, that is a good idea. I may put some lables in later.

Does anyone think an animation would be needed for the other types of piston valves? It would be relatively easy since I already got the basics set up. Would just have to change the shape of things.

I have some small issues with the force on the piston and the pressure behind it - nuances, really. The force on the piston rises as equilibrium is passed (rather than jumping), though is a bit of a jump as the forward area under pressure increases just after the piston moves and the volume behind the projectile is pressurized (though this volume does not quite get to chamber pressure due to flow, and the increasing volume behind the projectile). As the piston moves back the pressure behind it goes up a bit (not continuously down), as the volume is decreasing faster than the trigger valve can drain it. The piston will decelerate some before it gets fully open due to this trapped cushion of air. In fact, depending on the actual sizes the piston can bounce off this cushion of air and close again before the projectile is out of the barrel, in a really extreme case.

But these are small secondary effects, the overall sequence looks very good.